1. Technical Field
The present invention relates to wireless communication systems and, in particular, to resource allocation for 4G MIMO uplinks.
2. Description of the Related Art
Orthogonal frequency-division multiple-access (OFDMA) has been selected as the air-interface for 4G wireless networks operating over wideband multi-path fading channels. Classical OFDMA involves assigning non-overlapping subcarriers among different users. Consequently, resource allocation for the OFDMA networks has been the subject of intense research in recent years, with the most of the focus being on the downlink. In addition, most of the resource allocation problems hitherto considered have been formulated as continuous optimization problems. These problems are in general non-linear and non-convex. The IEEE 802.16m standard has selected OFDMA as its uplink air-interface and also allows multi-antenna transmission and reception in the uplink. However, resource allocation algorithms for the 802.16m OFDMA uplink have to be re-designed in order to account for finite constellations, codebook based precoding by multi-antenna users, and an equal power constraint that is enforced on all scheduled users.
The 3rd Generation Partnership Project long-term evolution Advanced (LTE-A) standard, meanwhile, has chosen discrete Fourier transform (DFT) spread OFDMA (DFT-S-OFDMA) as its uplink air-interface. DFT-S-OFDMA is a modified form of OFDMA wherein each user spreads its (coded) modulated information symbols using a DFT matrix and the spread symbols are mapped to its allocated subcarriers. The main advantage of the spreading operation is that, with a contiguous mapping, the spreading operation can result in a considerably lower peak-to-average-power ratio (PAPR) compared to classical OFDMA. In LTE-A systems the strict contiguity restriction of LTE systems is relaxed and each user can be allocated up-to two mutually non-contiguous frequency chunks (FCs), where each FC is a set of contiguous subcarriers. In addition, unlike LTE, the LTE-A uplink allows for multi-antenna transmission and reception. Resource allocation for the DFT-S-OFDMA uplink has been relatively much less studied. The LTE uplink scheduling problem itself is known to be NP-hard. Previous attempts have used a constant-factor approximation algorithm and a message passing based algorithm. However, neither of these algorithms is applicable to the LTE-A uplink.